This invention relates to a decoding device for generating digital signals and reliability of the digital signals from reproduced signals or received signals of coded digital signals and in particular to a decoding device for generating reliability from said reproduced signals or received signals of coded digital signals for every symbol consisting of a plurality (n) of binary signals.
In a usual digital system, digital signals are transmitted or recorded with parity signals for error correction. Errors which have occured at the transmission (sending/receiving) or at the recording/reproduction are corrected on the basis of the parity signals and thereafter the original digital signals are restored.
The hard decision decoding and the soft decision decoding are known as the error correcting method.
By the hard decision decoding a signal is made to correspond to a binary signal, "0" or "1", depending on whether the output voltage of the received (reproduced) signal is higher or lower than a predetermined reference voltage. For the binary signal of a digital system thus obtained, the errors are corrected on the basis of the parity signals for error correction and decoded to the original one. This method is widely utilized in compact disc players and others, because the decoding device can be easily constructed.
On the other hand, by the soft decision decoding, e.g. as indicated in FIG. 2, when the received (reproduced) output voltage V synchronized with the clock signal c is higher than +V.sub..theta., the output signal is "1"; when it is lower than -V.sub..theta., the output signal is "0"; and when it is comprised between +V.sub..theta. and -V.sub..theta., the output signal is "e" (erased). This method is disclosed e.g. in JP-A- 58-90853. Further, in JP-B-60-2812 is disclosed a soft decision decoding device for detecting errors, in which, only when the error position is in accordance with the erasure "e" stated above, the error is corrected and when the error position is not in accordance therewith, it is thought that an error, which is beyond the correcting capacity, is produced. The known soft decision decoding device described above is one of the simplest examples and in general the soft decision decoding is one, by which a binary signal and a reliability representing the likelihood of the value of the binary signal are obtained and the errors in the digital signal are corrected by using both this reliability and parity signals for error correction. For this reason the latent capacity, which the error correction code has, can be amply brought out and the signal-noise ratio (SN ratio) can be increased by about 2-3 dB with respect to that of the hard decision decoding.
The prior art techniques relate to a decoding device for generating a digital signal and its reliability for every digital signal from the reproduced or received signal and nothing is disclosed to generate the reliability for every symbol consisting of a plurality (n) of digital signals of coded digital signals of 2.sup.n - ary codes. For this reason it was not possible to apply the soft decision decoding for 2.sup.n - ary codes, for which errors are corrected for every symbol, such as e.g. codes over GF(2.sup.n). GF(2.sup.n) means Galois Field, having 2.sup.n elements.